Disjunction of the sex chromosomes in spermatogenesis in mammals has been previously assumed to result from homologous synapsis of a portion of the X with a portion of the Y, followed by crossing over and chiasmata formation. An alternate hypothesis is presented in this proposal: the observed synapsis between the X & Y in most mammals is non-homologous, the "precocious" desynapsis observed during early pachytene has evolved to minimize crossing over between the two, and an achiasmatic end association holds them together until anaphase I, thereby assuring normal disjunction. Sex reversal, (Sxr) a "gene" in mice has been reported to involve an exchange (a normal crossover between homologous regions) of a normal X and a rearranged Y. XY, Sxr mice will be used to test whether Y transmission of genes on the distal end of the X which have never been observed to cross over with the Y can be demonstrated in these animals. The sites (or lack thereof) of crossover in XY, Sxr mice and normal sibs labeled during meiosis with BrdU and FPC stained will be compared cytologically. EM analysis of microspread pachytene spermatocytes will be used to determine whether or not synapsis of the XY in XY, Sxr is homologous or nonhomologous and whether or not timing of meiotic events in altered in XY, Sxr mice (i.e. is desynapsis delayed so that crossing over, which has been shown to occur during the later part of pachytene, has more opportunity to take place?). In addition, the tendency of XY, Sxr males to a high frequency of univalents at pachytene (50%) and metaphase I (90% and a four fold increase in frequency of XMO progeny will be utilized to determine whether or not O and XY primary spermatocytes survive until Metaphase II, or whether XMO progeny arise through loss of the paternal sex chromosome in early embryonic cleavage.